Conventionally, there is a discharge lamp ballast configured to convert the an input DC power into an AC power and to light a high-intensity discharge lamp such as a HID lamp (High-intensity discharge lamp). In a related-art discharge lamp ballast 90 shown in FIG. 8, a DC-DC converter circuit 91 serving as a DC power converter circuit converts a DC voltage of a DC power supply PS into a DC power, and then an inverter circuit 92 converts the DC power into a low-frequency AC power and then supplies the output to a discharge lamp La via a starter circuit 93.
The DC-DC converter circuit 91 is of a fly-back converter system. The DC-DC converter circuit 91 controls the DC power supplied to the discharge lamp La acting as a load, by adjusting a PWM signal (Pulse Width Modulation signal) for driving a switching element Q0 connected in series to a primary winding of a transformer T.
The inverter circuit 92 has a full bridge configuration including switching elements Q1 to Q4. By alternately turning ON/OFF the paired switching elements Q1, Q4 and the paired switching elements Q2, Q3, the inverter circuit 92 converts the DC power fed from the DC-DC converter circuit 91 into a rectangular AC power.
In the starter circuit 93, a pulse driver circuit 931 provided on a primary side of a pulse transformer PT supplies a pulse current at a start time. Accordingly, a high voltage produced on a secondary side in accordance with a turn ratio of a coil is applied to the discharge lamp La, thereby starting an electric discharge of the discharge lamp La.
In the discharge lamp ballast 90 configured in this manner, the rectangular low-frequency AC power is supplied from the inverter circuit 92 to the discharge lamp La in order to avoid an acoustic resonance phenomenon and also to suppress electrode wear and a cataphoresis phenomenon. However, when the AC power is supplied, a lamp current passes through a zero point when a polarity of the AC power is reversed. Therefore, the electric discharge is stopped at the moment that the polarity of the lamp current is reversed.
In order to start the flow of electric current in the opposite direction after the lamp current is reversed from the zero, normally it is necessary to apply a predetermined high voltage called a reignition voltage to the discharge lamp La.
As shown in FIG. 9, when an output voltage Vo of the inverter circuit 92 is reversed, a lamp current Ila also starts to be reversed. Due to an inductance component (series inductance) Lp on the secondary side of the pulse transformer PT of the starter circuit 93, the lamp current Ila cannot change so sharply as the output voltage Vo, and is reversed to have a predetermined gradient dIla/dt.
The reignition voltage is increased as the gradient dIla/dt of the lamp current Ila at a time of polarity reversal is decreased. When the necessary reignition voltage is not supplied from the inverter circuit 92, a time Tzw (referred to as a “zero current period” hereinafter) in which the lamp current Ila becomes zero or is maintained to an electric current lower than an ordinary current occurs, as shown in FIG. 10. Thus, the noise may be generated, or the life of the discharge lamp La may be badly affected. Also, when the zero current period Tzw is extended much more, the flickering or the going-out of an illumination light is caused.
The zero current period Tzw caused at the polarity reversal of the lamp current Ila by decreasing the reignition voltage can be suppressed by reducing the inductance component Lp of the starter circuit 93 thereby increasing the gradient dIla/dt at the polarity reversal. However, there is a limit to the reduction of the inductance component Lp in terms of the starting performance.
For this reason, in the related-art discharge lamp ballast 90 shown in FIG. 8, by a method described below, an output of the DC-DC converter circuit 91 at the polarity reversal is increased, and thus the output voltage Vo of the inverter circuit 92 is increased, so that the necessary reignition voltage is maintained.
In the discharge lamp ballast 90, a dead time Td in which all switching elements Q1 to Q4 are turned OFF is set in order to prevent a short-circuited state of the circuit due to simultaneously ON state of the switching elements Q1, Q2 and Q3 and Q4 when the pair of switching elements Q1, Q4 and the pair of switching elements Q2, Q3 of the inverter circuit 92 are turned ON/OFF alternately. Therefore, a dead time adding circuit 941 is provided in an inverter driving signal generator circuit 94.
In the period of the dead time Td, a PWM signal generator circuit 96 is supplied not with a PWM command signal output from an error amplifier 953 of an output feedback control circuit 95 but with a predetermined command signal 981 for generating an output larger than the ordinary output. According to the command signal 981, as shown in FIG. 11, an output voltage V2 of the DC-DC converter circuit 91 is increased.
As a result, the output voltage Vo of the inverter circuit 92 is increased immediately after the start of the reversal whereby the necessary reignition voltage is maintained. Further, the gradient dIla/dt at the polarity reversal of the lamp current Ila can be increased by increasing the output voltage Vo (for example, see Patent Document 1).
In this method, when the polarity is reversed, a time Tt is shortened. The time Tt is from a time at which the lamp current Ila is in the polarity before the reversal of the lamp current Ila to a time at which the lamp current reaches zero. However, the DC-DC converter circuit 91 executes a power conversion based on the switching action, and thus the output of the DC-DC converter circuit 91 is not increased immediately after the PWM operating conditions (switching conditions) are changed. In particular, in the case of the DC-DC converter circuit 91, such as the fly-back converter, the step-up/down chopper, or the like, configured to accumulate an energy in the circuit elements when the ON condition of the switching element Q0 and then to discharge the accumulated energy to the load side when the OFF condition of the switching element Q0, the output voltage is increased stepwise every time of switching. As a result, the time Tt required until the lamp current Ila reaches zero is be shortened, and thus this time Tt comes close to a switching period Tsw of the DC-DC converter circuit 91 (for example, Tt≦3·Tsw).
At this time, the number of times of switching during the time Tt required until the lamp current Ila reaches zero may be decreased, and thus it may become difficult to obtain the output voltage Vo of the inverter circuit 92 which ensures the necessary reignition voltage.
The number of times of timings, i.e., OFF-timings, at which the output voltage Vo is increased during the time Tt required until the lamp current Ila reaches zero, is changed depending on the case where the reversing operation is started when the switching element Q0 of the DC-DC converter circuit 91 is turned ON or the case where the reversing operation is started when this switching element Q0 is turned OFF. In the former case, the output voltage Vo of the inverter circuit 92 in the zero current period Tzw, in which the lamp current Ila is maintained at zero, is decreased, and thus it may become difficult to ensure the necessary reignition voltage.